I located a stud, but the drywall floats an inch above it, plus the drywall itself is another 1/2 inch. So a couple 3-inch screws (and not a micron longer or they'll protude into my nice old neighbor's kitchen!) is required.

Pretending the drywall won't hold any weight, that means the 3-inch screws will be half-in and half-out of the wood. With a TV hanging off the end of them.

Recipe for disaster?

+++++++++++++++++++

So, three days hence, here's what I did: instead of using the living room wall I share with my neighbor, I used the living wall room that I share with my own hall closet (which wasn't my original choice due to window/sun glare).

I now have several 6" bolts going through the drywall, plaster & lathe, and studs in the closet, with big-ass washers for a little extra "spread". The Cheetah mount is wide & flat enough that it isnt pulling into or denting the drywall, and amazingly there is ZERO glare/reflection coming off the window.

Thanks everyone for your help, you gave me the info, perspectives, vocabulary and confidence to tackle this problem intelligently. :)

I'd curious as to what's actually behind your wall and would probably be inclined to cut a hole behind the mount in order to have a look. You could also put the hole slightly below the mount and use it to route cables through the wall.
–
StevenOct 13 '12 at 2:48

3 Answers
3

Case 1: Bridge Air Gap By Fastening to Building Structure With Overhung-load Bearing Fasteners

The air gap problem is quite common: The sheetrock on many walls I have encountered doesn't contact the studs everywhere. So, when you tighten down a TV mount to the wall, the drywall deflects toward the studs. You can easily cause cracking of the drywall, particularly when there is a seam in the area.

The principle of the solutions in this case make use of a fastener's ability to support an overhung load.

The fastener and the building structure have to be strong enough to withstand the torque created by the downward force of TV and Mount acting over the distance between the drywall surface and the building structure.

Case 1.1: Use Hanger Bolts

Assuming you find wood building structure in there (furring doesn't count here because it will twist under the torque), I recommend hanger bolts.

Use a spade bit to clear a hole in the drywall big enough to accommodate a nut flush with the surface of the drywall. Then put the mount over the hanger bolt and tighten another nut down on that. The first nut will keep the drywall from deflecting toward the wall. Size the hanger bolt larger than normal because of the overhung load. From your description I would guess that 4x 1/4" hanger bolts would be adequate.

Case 1.2: Variation for Concrete Construction

I've also encountered furred out concrete clad in drywall (common in highrise condominiums). In that case, the same principal as Case 1.1 applies, but use "drop-in anchors" and a threaded stud instead of hanger bolts.

Case 2: Fasten to Furring with No Air Gap

Sometimes drywall is attached to and is in contact with (i.e. no air gap) furring.

Case 2.1 Furring Attached to Building Structure

Where furring is attached directly to the building structure, drive multiple wood screws directly into the furring. If there is some hard material behind the furring, ensure that the screws are short enough. Otherwise they can jack the furring from the building structure when their points impact cinderblock or concrete.

The principle of this solution is to transfer the load to the building's structure directly through the furring.

Case 2.2 Furring Dubiously Attached to Building Structure

Where furring and drywall are creating their own structure, drive multiple wood screws into the furring to fasten your TV mount.

The principle of this solution is to use the furring to distribute the load to a large area of the composite structure of furring and drywall.

Drywall itself is rather heavy, so if the wall is able to hold itself up, there's a good chance it can hold up your TV to provided you distribute the load over a large enough area of drywall and furring.

+1 for a good solution, assuming there is adequate structure below. I worry that, if it is strapping, there is not enough body for the hanger bolt approach. Per your discussion of plaster/strapping, heavy duty toggle type bolts (several) might be sufficient, but it's a risk.
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bibAug 14 '12 at 15:00

@bib Agreed. Updated answer to advise against using strapping to support overhung load.
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alx9rAug 14 '12 at 18:07

@bib Entire case created for drywall/strapping without building structure.
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alx9rAug 14 '12 at 18:08

My concern is the "float." If the drywall is an inch "above" the stud, what is the drywall attached to? Not the stud. Also, standard construction has a 2x4 stud with its short side facing the finish material and its long side, about 3.5 inches, perpendicular to the finish material. If you have only 1.5 inches before poking through, this is a very non-standard structure. Also, even if the stud were flat (the 3.5 inch side facing the room) your 3 inch screw, run in 1.5 inches, should just be touching the back of your neighbor's finish material.

Mounts are intended to have the majority of thrust perpendicular to the wall. This exerts a shear force on the screws, rather than a horizontal pulling force. With shallow flat screens and flush (not angled or pivot) mounts, that is primarily true.

But that is the majority, not the sole force. There is always some lateral force, tending to pull the top out from the wall, and push the bottom in. If the drywall is floating, there is a serious risk that the mount will push in at the bottom and not have anything to keep it from punching through the drywall.

You really should try to find a solid point of mounting. There may be some other wood somewhere that is between the drywall and the "studs" you found. I would suggest that you keep searching.

"Float" is not a standard building term for this, just my characterization. Float is used for other things in DIY. But also see BMitch's answer as well. Need a bit more info. An electronic stud finder or a series of very small drilled holes may reveal more structure.
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bibAug 12 '12 at 23:30

2

Sounds like it's shared wall and it's an attempt at soundproofing (studs are staggered so sheetrock isn't attached at both sides). If that's the case, then technically you're screwing into your neighbors studs, which is going to increase sound-trasfer. I'd suggest choosing a different wall.
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DA01Aug 14 '12 at 6:15

Having the drywall floating an inch off the studs sounds like you have some kind of sound attenuation channel installed to keep the studs. These are used to prevent sound from traveling between the drywall and studs, either because it's a common wall with a neighbor in a multi-unit structure, or because you live near a loud area (such as a highway).

The drywall itself, on these channels, shouldn't be used to support a heavy load. And putting a screw through the drywall into the studs short circuits the separation allowing sound to travel easier. If it's a common wall, you may be violating some rental or HOA agreement. And if you live near a noise source, it's eliminating whatever benefit they gave you when building the house this way.

For the screw itself, first, it really should be a bolt that has more resistance to shear and bending. The further out from the wall, the more leverage the weight has to bend the screw/bolt. And as blb points out, you can cause the drywall at the bottom of the mount to collapse back onto the studs.

If that is some type of sound attenuation channel, then my suggestion would be to mount the TV free standing (on top of a stand) or locate it on another wall.

Edit: The comment about "prewar" makes me suspect that you have wood strapping to attach the drywall to a flat surface. The strapping is often only an inch or so wide. Find how they ran the strapping and run your bolts through the drywall, strapping, even the plaster behind. If you can catch some structure behind the plaster, that's even better, but you should have enough to support a flatscreen.

+1 for coming up with a possible explanation for the float (we're city mice - we just put up with the noise). I am still suspicious of the gap and the shallow stud depth.
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bibAug 12 '12 at 23:26

The drywall is about 4" over old plaster & lathe (it's a prewar bldg), and the "odd stud" I'm looking at I only found because there's an outlet box attached to it. I'm going to remove the box tomorrow (yes i'll turn off the breaker here first!) and try to peak inside the wall, instead of continuing my comically unsuccessful tap-test that has resulted in four useless holes in my once beautiful wall... thanks again :)
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jrocknycAug 12 '12 at 23:43

Per BMitch - strapping is not framing, so lets try to find something that can hold up something in addition to itself.
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bibAug 13 '12 at 0:45

@bib While it's not ideal, strapping does a nice job spreading the load over a wide area of drywall. I wouldn't categorically rule out using the strapping/drywall composite for bearing load, particularly if it is type "X" fire grade drywall which is remarkably sturdy. I've done this successfully where it was the least bad option.
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alx9rAug 14 '12 at 6:04